Planetary Evolution, Habitability and Life

Planetary Evolution, Habitability and Lif

What mechanisms dominate the activity of Geminid Parent (3200) Phaethon?

A long-term sublimation model to explain how Phaethon could provide the Geminid stream is proposed. We find that it would take $\sim6$ Myr or more for Phaethon to lose all of its internal ice (if ever there was) in its present orbit. Thus, if the asteroid moved from the region of a 5:2 or 8:3 mean motion resonance with Jupiter to its present orbit less than $1$ Myr ago, it may have retained much of its primordial ice. The dust mantle on the sublimating body should have a thickness of at least $15$ m but the mantle could have been less than $1$ m thick $1000$ years ago. We find that the total gas production rate could have been as large as $10^{27}\rm~s^{-1}$ then, and the gas flow could have been capable of lifting dust particles of up to a few centimeters in size. Therefore, gas production during the past millennium could have been sufficient to blow away enough dust particles to explain the entire Geminid stream. For present-day Phaethon, the gas production is comparatively weak. But strong transient gas release with a rate of $\sim4.5\times10^{19}\rm~m^{-2}s^{-1}$ is expected for its south polar region when Phaethon moves from $0^\circ$ to $2^\circ$ mean anomaly near perihelion. Consequently, dust particles with radii of $<\sim260~\mu m$ can be blown away to form a dust tail. In addition, we find that the large surface temperature variation of $>600$ K near perihelion can generate sufficiently large thermal stress to cause fracture of rocks or boulders and provide an efficient mechanism to produce dust particles on the surface. The time scale for this process should be several times longer than the seasonal thermal cycle, thereby dominating the cycle of appearance of the dust tail.Comment: 10 pages, 5 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Societ

Early Thermal Evolution of Planetesimals and its Impact on Processing and Dating of Meteoritic Material

Radioisotopic ages for meteorites and their components provide constraints on the evolution of small bodies: timescales of accretion, thermal and aqueous metamorphism, differentiation, cooling and impact metamorphism. Realising that the decay heat of short-lived nuclides (e.g. 26Al, 60Fe), was the main heat source driving differentiation and metamorphism, thermal modeling of small bodies is of utmost importance to set individual meteorite age data into the general context of the thermal evolution of their parent bodies, and to derive general conclusions about the nature of planetary building blocks in the early solar system. As a general result, modelling easily explains that iron meteorites are older than chondrites, as early formed planetesimals experienced a higher concentration of short-lived nuclides and more severe heating. However, core formation processes may also extend to 10 Ma after formation of Calcium-Aluminum-rich inclusions (CAIs). A general effect of the porous nature of the starting material is that relatively small bodies (< few km) will also differentiate if they form within 2 Ma after CAIs. A particular interesting feature to be explored is the possibility that some chondrites may derive from the outer undifferentiated layers of asteroids that are differentiated in their interiors. This could explain the presence of remnant magnetization in some chondrites due to a planetary magnetic field.Comment: 24 pages, 9 figures, Accepted for publication as a chapter in Protostars and Planets VI, University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C. Dullemond, Th. Hennin

The habitability of stagnant-lid Earths around dwarf stars

The habitability of a planet depends on various factors, such as delivery of water during the formation, the co-evolution of the interior and the atmosphere, as well as the stellar irradiation which changes in time. Since an unknown number of rocky exoplanets may operate in a one-plate convective regime, i.e., without plate tectonics, we aim at understanding under which conditions planets in such a stagnant-lid regime may support habitable surface conditions. Understanding the interaction of the planetary interior and outgassing of volatiles with the atmosphere in combination with the evolution of the host star is crucial to determine the potential habitability. M-dwarf stars in particular possess a high-luminosity pre-main sequence phase which endangers the habitability of planets around them via water loss. We therefore explore the potential of secondary outgassing from the planetary interior to rebuild a water reservoir allowing for habitability at a later stage. We compute the boundaries of the habitable zone around M, K, G, and F-dwarf stars using a 1D cloud-free radiative-convective climate model accounting for the outgassing history of CO2 and H2O from an interior evolution and outgassing model for different interior compositions and stellar luminosity evolutions. The outer edge of the habitable zone strongly depends on the amount of CO2 outgassed from the interior, while the inner edge is mainly determined via the stellar irradiation, as soon as a sufficiently large water reservoir has been outgassed. A build-up of a secondary water reservoir for planets around M-dwarf stars is possible even after severe water loss during the high luminosity pre-main sequence phase as long as some water has been retained within the mantle. Earth-like stagnant-lid planets allow for habitable surface conditions within a continuous habitable zone that is dependent on interior composition.Comment: 15 pages, accepted by A&A, abstract shortene

Land Fraction Diversity on Earth-like Planets and Implications for their Habitability

A balanced ratio of ocean to land is believed to be essential for an Earth-like biosphere and one may conjecture that plate-tectonics planets should be similar in geological properties. After all, the volume of continental crust evolves towards an equilibrium between production and erosion. If the interior thermal states of Earth-sized exoplanets are similar to the Earth's, one might expect a similar equilibrium between continental production and erosion to establish and, hence, a similar land fraction. We will show that this conjecture is not likely to be true. Positive feedback associated with the coupled mantle water - continental crust cycle may rather lead to a manifold of three possible planets, depending on their early history: a land planet, an ocean planet and a balanced Earth-like planet. In addition, thermal blanketing of the interior by the continents enhances the sensitivity of continental growth to its history and, eventually, to initial conditions. Much of the blanketing effect is however compensated by mantle depletion in radioactive elements. A model of the long-term carbonate-silicate cycle shows the land and the ocean planet to differ by about 5 K in average surface temperature. A larger continental surface fraction results both in higher weathering rates and enhanced outgassing, partly compensating each other. Still, the land planet is expected to have a substantially dryer, colder and harsher climate possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth. Using a model of balancing water availability and nutrients from continental crust weathering, we find the bioproductivity and the biomass of both the land and ocean planet to be reduced by a third to half of Earth's. The biosphere on these planets might not be substantial enough to produce a supply of free oxygen

Studying Io's Volcanic History Using Thermal Infrared Measurements

A new thermal infrared instrumentation to observe Io combined with the unique capabilities of PEL will provide new insights into the evolution of Io

Differentiation of Vesta: Implications for a shallow magma ocean

The Dawn mission confirms predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and confirms Vesta as the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid. We have expanded the model of Neumann et al. (2012) that includes accretion, compaction, melting and associated changes of material properties and partitioning of 26Al, advective heat transport, and differentiation by porous flow, to include convection and effective cooling in a magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by porous flow or by convection and heat flux of a magma ocean with a high effective thermal conductivity. We show that partitioning of 26Al and its transport with the silicate melt is crucial for the formation of a magma ocean. Due to the accumulation of 26Al in the sub-surface (for formation times t0<1.5 Ma), a shallow magma ocean with a thickness of 1 to a few tens of km (depending on the silicate melt viscosity) forms. The lifetime of the shallow magma ocean is O(10^4)-O(10^6) years and convection in this layer is accompanied by the extrusion of 26Al at the surface. The interior differentiates from the outside inward with a mantle that is depleted in 26Al and core formation is completed within ~0.3 Ma. The lower mantle experiences melting below 45% suggesting a harzburgitic to dunitic composition. Our results support the formation of eucrites by the extrusion of early partial melt and cumulative eucrites and diogenites may form from the crystallizing shallow magma ocean. Silicate melt is present for up to 150 Ma, and core convects for ~100 Ma, supporting the idea of an early magnetic field.Comment: 57 pages, 13 figures, 2 table